This proposal aims to determine the role of natriuretic peptide receptor-C (NPR-C) in mitigating acute lung injury. Diseases such as pneumonia, congestive heart failure, and acute respiratory distress syndrome (ARDS) are characterized by movement of fluid, protein and cells from the pulmonary circulation to the interstitial and alveola space. This transudation of vascular fluid impairs gas exchange and initiates a cascade of inflammatory events that leads to acute lung failure. Cell signaling pathways that regulate fluid and cellular transudation across the pulmonary vascular membrane play critical roles in determining the extent of lung damage that occurs and the time it takes for the lung to recover. The natriuretic peptides (NP) are a family of proteins that play critically important roles in maintaining appropriate amounts of fluid in the circulation. NPs are released from the heart in response to increased blood volume and act to increase the permeability of blood vessels, thereby facilitating transudation of fluid, solutes and proteins from the intravascular to the interstitial space of systemic vascular beds. In the pulmonary circulation, however, NPs appear to protect against increased acute lung injury and pulmonary edema formation. In fact, numerous studies have shown that atrial natriuretic peptide (ANP) blunts acute lung injury in animal models and clinical studies. This differential effect of NPs on vascular permeability in the systemic and pulmonary vascular beds may be explained by differences in effects of the NP-receptors. Previous studies by other investigators suggest that enhancement of vascular permeability is mediated by NPR-A. However, recent studies from our laboratory suggest that the protective effects of NPs on acute lung injury are mediated by NPR-C. This proposal will test the hypothesis that NPR-C protects against acute lung injury by determining if lung injury is worse in mice lacking NPR-C and if NPs fail to protect against lung injury in NPR-C deficient mice. The proposal will also determine if barrier function in pulmonary vascular endothelial cells can be altered by increasing or decreasing NPR-C expression. Other studies aim to elucidate downstream signaling pathways responsible for the protective effects of NPR-C by determining if the protective effect of NPR-C is mediated by activation of G-inhibitory protein (Gai) and downstream modulation of cAMP. Finally, animal studies will be performed to determine if increased expression of NPR-C in vivo can protect against acute lung injury. Pulmonary edema formation is the primary cause of morbidity and mortality in severe cases of acute respiratory failure. Identification of receptors and their associated signal transduction pathways responsible for maintenance of intact pulmonary endothelial barrier function is vital to furthering our understanding of pulmonary permeability and developing new therapies for patients with acute lung injury.